suggest, two images are correlated directly in the first 
type; in the second type an image and matched filter 
are used as inputs. The matched filter is a Fourier 
transform recording of the second image. In the 
image/image correlator, the correlation signal appears 
as a light amplitude on the optical axis in the output 
plane of the correlator. It can be measured with a 
small aperture and phototube arranged in tandem; the 
detector configuration remains stationary. The move- 
ment of either image is required to determine when 
alignment is achieved. On the other hand, the output 
format of the image/matched-filter correlator is such 
that the correlation function is a light amplitude 
distribution that moves in the output plane whenever 
the image is displaced; the matched filter must remain 
fixed in the optical system. A typical detector for this 
type of correlator would be one that can scan the 
correlator output plane, for example, a vidicon, orthi- 
con, or image dissector. 
A comparison of the two correlator types shows 
that the image/matched-filter correlator is more suit- 
able for image search problems while the point trans- 
fer problem, stereo image matching, can be handled 
more accurately and conveniently with the image/ 
image correlator. Since we are interested primarily in 
image matching, the subsequent discussion deals with 
the image/image correlator. 
IMAGE/IMAGE OPTICAL CORRELATOR 
The elements of the image/image correlator are 
shown in Figure 6. The system contains a laser light 
source, a phototube detector, three transform lenses, 
and two spatial filters. The laser provides an intense 
beam of collimated coherent light that acts as the 
signal carrier through the optical system. It is directed 
down the optical axis of the system towards the 
phototube and successively passes through each ele- 
ment in the optical train. 
As the beam emerges from the first diapositive, it 
picks up all the pictorial information stored in the 
image. The light fans out as it proceeds towards the 
transform lens. This is the diffraction effect produced 
by the detail in the diapositive; it carries all the perti- 
nent image information. At most, only one-half of 
the transmitted light can be diffracted. 
The transform lens operates on this incident light 
in a specific manner. It collects the light according to 
the angle of incidence and directs all the rays having 
the same angle to a common point in its front focal 
plane. This is illustrated by the two cylinders of light 
emerging from the diapositive. Every ray in each 
cylinder has the same incidence angle as it enters the 
transform lens. The emerging rays that were originally 
parallel to the optical axis intersect on the optical 
COMPARISON OF CORRELATION TECHNIQUES 
Laser Light 
Source 
Stereo Diapositive — 1 
    
        
Lens Stereo Diapositive — 2 
Transform Fourier- Frequency 
Lens " Plane— 1 
oi} DC Block 
Jr 
Nd a 1 Imaging 
T ar^ 
Fourier Frequency 
Original Spatial rd i Plane—2 
Ima Filterin i > ESS 
v s = 17 = 3 Correlation- Light 
3 nm A, Signal 
(Qr ; : fr Phototube 
Pussies) Pin Hole  Correlation- Electrical 
image Filter Signal 
Figure 6 Basic Coherent Optical Image/Image 
Correlator 
axis, while those having the specific angle illustrated 
intersect to the left of the optical axis in the front 
focal plane of the lens. The lens operates on each 
bundle of diffracted light in the same manner. The 
intersection point of each group of rays is defined by 
the principal ray. 
It can be shown mathematically that the angle of 
the diffracted light is directly related to the spatial 
frequency of the image detail. The transform lens 
thus distributes the diffracted light in a manner that 
is analogous to the Fourier frequency decomposition 
of signals. Because of this analogy, the front focal 
plane of the transform lens is called the Fourier plane 
(or sometimes just the frequency plane). The ampli- 
tude of the light at any specific point in the Fourier 
plane represents the amount of specific size detail in 
the image, that is, the specific spatial frequency con- 
tent of the imagery. 
The Fourier transform property of a lens provides 
a means by which the structure of an image can be 
modified by removing some of its spatial frequencies. 
The removal of light is illustrated by placement of an 
opaque circular light block on the optical axis. This in 
reality is a high-pass spatial filter removing the direct 
component (dc) from the image. It takes out the 
average background in the image. The imaging lens by 
the nature of its transform property collects the light 
from the frequency plane and retransforms the fre- 
quency distribution to an inverted image. The inver- 
sion process does not affect the correlation scheme 
because the second image can be easily placed in an 
inverted position to compensate for the lens inver- 
sion. The effect of the dc block on the reconstructed 
image is depicted by the lower-left sketches that show 
a simple image and its dc-filtered reconstruction. 
Only the detail is preserved in the reconstructed 
image. 
The light emerging from the second stereo diaposi- 
tive has an amplitude distribution that is the product 
of the detail in image 1 and the unfiltered image in 
the second diapositive. The output lens takes another 
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